1. Field of the Invention
The present invention relates to an image forming apparatus configured to carry a toner image on a recording medium and then fix the toner image to the recording medium by heat to form an image and, more particularly, to an image forming apparatus arranged to optimize input energy to a fixing section according to conditions of output images.
2. Description of Related Art
An image forming apparatus using toner is conventionally configured to form a toner image and then transfer the toner image to a recording medium such as a printing sheet. The toner image transferred to the recording medium is thereafter fixed to the recording medium by heating. Meanwhile, in recent years, the image forming apparatus has been demanded for high-level energy saving. Herein, energy to be consumed by a fixing section arranged to heat a recording medium carrying thereon a toner image accounts for no small percentage of the total energy used by the image forming apparatus. Therefore, the image forming apparatus has strongly been demanded for energy saving of the fixing section.
One example of an image forming apparatus configured with energy saving of a fixing section is disclosed in Japanese patent application publication No. 2012-118496. In this image forming apparatus, a target fixing temperature in fixing processing is set changeable. This apparatus also includes a function of making gray-scale (gradation) processing of image information in image processing for image formation. The target fixing temperature is changed based on whether or not the image information to be subjected to image formation includes a content needing to undergo halftone processing. Furthermore, since the gray-scale processing includes various types, the target fixing temperature is changed according to the type of the gray-scale processing to be executed. The publication says this enables setting a best fixing temperature per one recording medium during continuous printing. The publication also says there is no need for selection of a specific mode at that time neither a huge amount of information.
However, the aforementioned image forming apparatus disclosed in the publication has the following problems. For energy saving of the fixing section, it is essential not to increase the fixing temperature beyond a required minimum temperature. The required minimum temperature is mainly determined by an amount of toner present on the recording medium. Parameters related to an image to be formed and most likely to influence the toner amount are density and size of an object to be drawn (a drawing object). Meanwhile, the image forming apparatus in the publication basically uses, as index for determining the target fixing temperature, the presence/absence of halftone processing and the type of gray-scale processing. However, those parameters are not always reflected adequately in the amount of toner on the recording medium. Thus, the image forming apparatus still has a room for improvement in optimization of the fixing temperature for the purpose of energy saving.
The present invention has been made to solve the above problems and has a purpose to provide an image forming apparatus configured to set a target fixing temperature adequately reflecting contents of an image to be formed, thereby forming an appropriate image while optimizing input energy to a fixing section.